Pluggable Disk Schedulers in GEOM
Robert Watson
rwatson at FreeBSD.org
Fri Jan 5 14:38:43 UTC 2007
On Fri, 5 Jan 2007, lulf at stud.ntnu.no wrote:
> Anyway, I'd like to research a bit on this topic to just see how much it
> does matter with different I/O scheduling for different purposes.
I think working on this is interesting, but the one caution I'd have is that
it's possibly to introduce serious priority inversions through any complex
scheduling scheme for I/O. In our VFS, I/O is frequently performed while
holding locks or things that act like locks -- for example, during a directory
lookup, while pulling an inode off the disk, etc. The I/O will be initiated
by one thread, but then other threads will end up waiting for it also. If
there is a naive mapping of initiating thread priority to I/O request
priority, then you can end up with high priority threads being blocked on a
low priority tasks, leading to nasty starvation effects, especially if the
scheduler allows indefinite waiting for I/O at a low priority. This, at a
rough approximation, is the problem that Kirk ran into when trying to rate
limit bgfsck I/O in the kernel: key vnode locks, such as directory vnode
locks, would be held across de-prioritized I/O, and high priority processes
would then block on the vnode locks. There are various ways to address this,
not least priority propagation (in which I/O priority is increased to match
the priority of the highest priority thread waiting on the I/O request), but I
wanted to make sure you had it on the list of design concerns.
Robert N M Watson
Computer Laboratory
University of Cambridge
More information about the freebsd-geom
mailing list